The auditory response of the listener to the sounds reproduced in the room is greatly influenced by the diffusion of sound waves emerging from the speaker. At high acoustic frequencies, therefore, there is a need to use a speaker having wide directional characteristics.
Conventionally, horn-type speakers have chiefly been used as high- and mid-frequency speakers. However, horn speakers have the drawback of being low in directivity inasmuch as the sound wave is radiated from the horn mouth as a plane wave. Furthermore, a low-frequency reproduction with a horn speaker needs a greater horn length, entailing the drawback that the speaker becomes larger in its entirety.
To eliminate these drawbacks, various means have heretofore been employed. For example, a multi-cellular horn is known which comprises a multiplicity of horns of the same type so arranged as to form a part of spherical surface with the mouth of the horn. Also known is a sectoral horn having flaring side walls and upper and lower walls vertically caved in toward the principal axis of the horn to abruptly constrict the sound passage to elevate the sound pressure and to increase the medium density at the constricted portion, thereby increasing the phase velocity of sound wave, such that the sound waves will be radiated and spread out from the horn mouth in the form of a sector. Attempts have also been made to use materials to disperse sound waves or acoustical lenses.
Although the above-mentioned multi-cellular horn achieves a remarkable improvement in directional characteristics, it is disadvantageous in being complex in construction, expensive and large-sized. Further with the sectoral horn which is relatively inexpensive and has improved directional characteristics, it is difficult to effect reproduction at low frequencies without adverse effect. In fact, an attempt to overcome this difficulty has entailed the drawback that the horn becomes large. Furthermore, acoustical lenses and the like are not only expensive but also large and necessitate an increased space.
Thus none of the conventional horn speakers are satisfactory to fulfil all the requirements in respect of directional characteristics, low impedance characteristics, compactness and cost.